So I know all the elements of $S_4=\{e,(12),(13),(14),(23),(24),(34),(12)(34),(13)(24),(12)(23),(123),(132),(124),(142),(134),(143),(234),(243),(1234),(1342),(1423),(1243),(1432),(1324)\}$
I also understand that $e, (12), (12)(34), (123)$, and $(1234)$ are the classes. What's confusing me is how to find the number of elements in each class. For example I see ($12)(12)(12)^{-1}=(12)$ but when we look at the next element we see $(13)(12)(13)^{-1}=((13)(1) (13)(2))$.
How do we simplfy this/ how are we using number theory here to get elements of these classes?